Semiconductor device having cam that stores address signals

ABSTRACT

Disclosed herein is an apparatus that includes a plurality of address registers each storing an address signal, a plurality of counter circuits each storing a count value corresponding to an associated one of the address registers, a first circuit cyclically selecting one of the address registers in response to a first signal, a second circuit selecting one of the address registers based on the count value of each of the counter circuits, and a third circuit activating a second signal when the first and second circuits select the same one of the address registers.

BACKGROUND

In a semiconductor device such as a DRAM (Dynamic Random Access Memory), concentration of access on the same word line may cause deterioration of information retention characteristics of memory cells connected to an adjacent word line. Therefore, in some cases, a refresh operation of the memory cells is performed in addition to a normal refresh operation to prevent information of the memory cells connected to the adjacent word line from being lost. This additional refresh operation is called “row hammer refreshing operation”.

The row hammer refresh operation is performed on word lines adjacent to word lines at which accesses are concentrated. In order to realize this operation, addresses of a plurality of word lines at which accesses are concentrated are stored in an address storing circuit and one of the addresses is read from the address storing circuit at a time of the row hammer refresh operation. However, when the row hammer refresh operation is to be performed, which one of the addresses stored in the address storing circuit is to be read is the issue to be handled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a semiconductor device according to an embodiment of the present disclosure.

FIG. 2 is a block diagram for explaining a configuration of a refresh control circuit.

FIG. 3 is a schematic diagram for explaining a relation between an address of a word line at which accesses are concentrated and addresses of word lines on which a row hammer refresh operation is to be performed.

FIG. 4 is a block diagram for explaining a configuration of a row hammer address storing circuit.

FIG. 5 is a block diagram for explaining a configuration of a control circuit.

FIG. 6 is a timing chart for explaining an operation of the control circuit.

FIGS. 7A and 7B are timing charts respectively showing an example where a plurality of refresh operations are performed in response to one refresh command.

FIG. 8 is an example of a circuit diagram of an LFSR circuit.

DETAILED DESCRIPTION

Various embodiments of the present invention will be explained below in detail with reference to the accompanying drawings. The following detailed description refers to the accompanying drawings that show, by way of illustration, specific aspects and embodiments in which the present invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the present invention. Other embodiments may be utilized, and structural, logical and electrical changes may be made without departing from the scope of the present invention. The various embodiments disclosed herein are not necessary mutually exclusive, as some disclosed embodiments can be combined with one or more other disclosed embodiments to form new embodiments.

FIG. 1 is a block diagram of a semiconductor device 10 according to an embodiment of the present disclosure. The semiconductor device 10 may be a DDR4 SDRAM incorporated in a single semiconductor chip, for example. The semiconductor device 10 may be mounted on an external substrate, for example, a memory module substrate or a mother board. As shown in FIG. 1, the semiconductor device to includes a memory cell array 11. The memory cell array 11 includes a plurality of word lines WL, a plurality of bit lines BL, and a plurality of memory cells MC arranged at intersections of the word lines WL and the bit lines BL. Selection of a word line WL is performed by a row address control circuit 12, and selection of a bit line BL is performed by a column decoder 13. A sense amplifier 14 is connected to a corresponding bit line BL and a pair of local I/O lines LIOT/B. The pair of local I/O lines LIOT/B is connected to a pair of main I/O lines MIOT/B via a transfer gate 15 functioning as a switch. The memory cell array 11 is divided into (m+1) memory banks including memory banks BANK0 to BANKm.

A plurality of external terminals included in the semiconductor device 10 include command address terminals 21, clock terminals 22, data terminals 23, and power-supply terminals 24 and 25. The data terminals 23 are connected to an I/O circuit 16.

A command address signal CA is supplied to the command address terminals 21. One of the command address signals CA supplied to the command address terminals 21, which relates to an address, is transferred to an address decoder 32 via a command address input circuit 31. Another one that relates to a command is transferred to a command control circuit 33 via the command address input circuit 31. The address decoder 32 decodes an address signal and generates a row address XADD and a column address YADD. The mw address XADD is supplied to the row address control circuit 12, and the column address YADD is supplied to the column decoder 13. Further, a command address signal CA that functions as a clock enable signal CKE is supplied to an internal clock generator 35.

Complementary external clock signals CK and /CK are supplied to the clock terminals 22. The complementary external clock signals CK and /CK are input to a clock input circuit 34. The clock input circuit 34 generates an internal clock signal ICLK based on the complementary external clock signals CK and /CK. The internal clock signal ICLK is supplied to at least the command control circuit 33 and the internal clock generator 35. The internal clock generator 35 is activated by the clock enable signal CKE, for example, and generates an internal clock signal LCLK based on the internal clock signal ICLK. The internal clock signal LCLK is supplied to the I/O circuit 16. The internal clock signal LCLK is used as a timing signal that defines a timing at which read data DQ is output from the data terminal 23 at the time of a read operation. In a write operation, write data is input to the data terminal 23 from outside. In the write operation, a data mask signal DM may be input to the data terminal 23 from outside.

Power-supply potentials VDD and VSS are supplied to the power-supply terminals 24. These power-supply potentials VDD and VSS are supplied to a voltage generator 36. The voltage generator 36 generates various internal potentials VPP, VOD, VARY, and VPERI, for example, based on the power-supply potentials VDD and VSS. The internal potential VPP is used mainly in the row address control circuit 12. The internal potentials VOD and VARY are used mainly in the sense amplifier 14 included in the memory cell array 11. The internal potential VPERI is used in many other circuit blocks.

Power-supply potentials VDDQ and VSSQ are supplied to the I/O circuit 16 from the power-supply terminals 25. Although the power-supply potentials VDDQ and VSSQ may be the same potentials as the power-supply potentials VDD and VSS supplied to the power supply terminals 24, respectively, the dedicated power-supply potentials VDDQ and VSSQ are assigned to the I/O circuit 16 in order to prevent propagation of power-supply noise generated in the I/O circuit 16 to another circuit block.

The command control circuit 33 activates an active signal ACT when an active command is issued, and activates a refresh signal AREF when a refresh command is issued. The active signal ACT and the refresh signal AREF are both supplied to the row address control circuit 12. The row address control circuit 12 includes a refresh control circuit 40. The refresh control circuit 40 controls a refresh operation for the memory cell array 11 based on the row address XADD, the active signal ACT, and the refresh signal AREF. The refresh control circuit 40 will be described in detail later.

When a read command is issued firm outside, following the active command, the command control circuit 33 activates a column selection signal CYE. The column selection signal CYE is supplied to the column decoder 13. In response to this signal, read data is read out from the memory cell array 11. The read data read from the memory cell array 11 is transferred to the (O circuit 16 via a read-write amplifier 17 and an FIFO circuit 18, and is output to outside via the data terminals 23.

FIG. 2 is a block diagram for explaining a configuration of the refresh control circuit 40.

As shown in FIG. 2, the refresh control circuit 40 includes a refresh counter 41, an ARM sample generator 42, a sampling circuit 43, a row hammer address storing circuit 44, an address convertor 45, and a refresh address selector 46. The refresh counter 41 generates a normal refresh address NRADD. The normal refresh address NRADD is incremented or decremented in response to an internal refresh signal IREF. The internal refresh signal IREF can be a signal activated plural times on the basis of the refresh signal AREF. The sampling circuit 43 samples the row address XADD at a timing when a sampling signal SMP generated by the ARM sample generator 42 is activated, and supplies the sampled row address XADD to the row hammer address storing circuit 44. The ARM sample generator 42 may activate the sampling signal SMP at a timing when the active signal ACT is activated for a predetermined number of times. Accordingly, an address VADD of a word line WL at which accesses are concentrated is supplied to the row hammer address storing circuit 44. As described later, the row hammer address storing circuit 44 stores a plurality of row addresses VADD. The row addresses VADD stored in the row hammer address storing circuit 44 are supplied to the address convertor 45. The address convertor 45 converts the row addresses VADD to generate row hammer refresh addresses +1ADD, −1ADD, +2ADD, and −2ADD.

The row hammer refresh addresses +1ADD and −1ADD are addresses of word lines WL adjacent to the word line WL having the row address VADD assigned thereto on the both sides. The row hammer refresh addresses +2ADD and −2ADD are addresses of word lines WL two lines away from the word line WL having the row address VADD assigned thereto on the both sides. For example, when word lines WL1 to WL5 are arranged in this order as shown in FIG. 3 and accesses are concentrated at the word line WL3, the row address VADD corresponds to the word line WL3, the row hammer refresh addresses −1ADD and +1ADD correspond to the word lines WL2 and WL4, respectively, and the row hammer refresh addresses −2ADD and +2ADD correspond to the word lines WL1 and WL5, respectively. In the word lines WL4, WL2, WL5, and WL1 to which the row hammer refresh addresses +1ADD, −1ADD, +2ADD, and −2ADD are respectively assigned, there is a possibility that the information storing performance of associated memory cells MC is decreased because accesses are concentrated at the word line WL3 adjacent thereto or two lines away therefrom. The normal refresh address NRADD and the row hammer refresh addresses +1ADD, −1ADD, +2ADD, and −2ADD are supplied to the refresh address selector 46.

The refresh control circuit 40 further includes a counter circuit 47, a comparing circuit 48, and a refresh state circuit 49. The counter circuit 47 increments or decrements a count value CV in response to the internal refresh signal IREF. The comparing circuit 48 receives the count value CV and activates a refresh state signal RHR State each time the count value CV reaches a predetermined value. The predetermined value can be changed with a mode signal MODE. Therefore, it suffices to set the predetermined value to a small value with the mode signal MODE when the frequency of the row hammer refresh operations is to be increased, and set the predetermined value to a large value with the mode signal MODE when the frequency of the row hammer refresh operations is to be decreased. The refresh counter 41 may temporarily stop an update operation of the normal refresh address NRADD when the refresh state signal RHR State is activated.

The refresh state signal RHR State is supplied to the refresh state circuit 49. The refresh state circuit 49 generates refresh selection signals NR, RHR1, and RHR2 on the basis of the internal refresh signal IREF and the refresh state signal RHR State.

The refresh state circuit 49 activates the refresh selection signal NR when the refresh state signal RHR State is in an inactive state. The refresh selection signal NR is a signal activated when the normal refresh operation is to be performed. In a case where the refresh selection signal NR is activated, the refresh address selector 46 selects the normal refresh address NRADD output from the refresh counter 41 and outputs the normal refresh address NRADD as a refresh address REFADD. When the refresh state signal RHR State is in an active state, the refresh state circuit 49 activates the refresh selection signal RHR1 or RHR2. The refresh selection signal RHR1 is a signal activated when the row hammer refresh operation is to be performed on the word lines WL2 and WL4 adjacent to the word line WL3 at which accesses are concentrated. In a case where the refresh selection signal RHR1 is activated, the refresh address selector 46 selects the row hammer refresh addresses +1ADD and −1ADD output from the address convertor 45 and outputs the row hammer refresh addresses +1ADD and −1ADD as the refresh addresses REFADD. The refresh selection signal RHR1 is supplied also to the row hammer address storing circuit 44. The refresh selection signal RHR2 is a signal activated when the row hammer refresh operation is to be performed on the word lines WL1 and WL5 two lines away from the word lines WL3 at which accesses are concentrated. In a case where the refresh selection signal RHR2 is activated, the refresh address selector 46 selects the row hammer refresh addresses +2ADD and −2ADD output from the address convertor 45 and outputs the row hammer refresh addresses +2ADD and −2ADD as the refresh addresses REFADD.

FIG. 4 is a block diagram for explaining a configuration of the row hammer address storing circuit 44.

As shown in FIG. 4, the row hammer address storing circuit 44 includes a plurality of address registers 50 to 57, a plurality of counter circuits 60 to 67, a comparing circuit 70, and a control circuit 80. While eight address registers 50 to 57 are illustrated in an example shown in FIG. 4, the number of address registers included in the row hammer address storing circuit 44 is not limited thereto. The row addresses XADD sampled by the sampling circuit 43 are stored in the address registers 50 to 57, respectively. The counter circuits 60 to 67 correspond to the address registers 50 to 57, respectively.

The comparing circuit 70 compares the input row address XADD with each of the row addresses XADD stored in the address registers 50 to 57. When the input row address XADD matches with any of the row addresses XADD stored in the address registers 50 to 57, the comparing circuit 70 activates a corresponding one of hit signals HIT0 to HIT7. When any of the hit signals HIT0 to HIT7 is activated, the control circuit 80 increments the count value of a corresponding one of the counter circuits 60 to 67. Therefore, the count values of the counter circuits 60 to 67 indicate the numbers of times when the row addresses XADD stored in the address registers 50 to 57 are sampled by the sampling circuit 43, respectively. The control circuit 80 includes a minimum pointer 81 that indicates one of the counter circuits 60 to 67 having a smallest count value, and a maximum pointer 82 that indicates one of the counter circuits 60 to 67 having a greatest count value.

On the other hand, when none of the hit signals HIT0 to HIT7 is activated, that is, when the input row address XADD does not match with any of the row addresses XADD respectively stored in the address registers 50 to 57, the control circuit 80 resets one of the counter circuits 60 to 67 indicated by the minimum pointer 81 to an initial value and supplies a point number MIN to the address registers 50 to 57. Accordingly, the input row address XADD is overwritten in one of the address registers 50 to 57 indicated by the point value MIN. In this way, when the input row address XADD does not match with any of the row addresses XADD respectively stored in the address registers 50 to 57, the value of one of the address registers 50 to 57 storing the row address XADD that is least frequently accessed is overwritten.

One of the row addresses XADD stored in the address registers 50 to 57 is output as the row address VADD in response to the refresh selection signal RHR1. The control circuit 80 further includes a sequential counter 83. When the refresh selection signal RHR1 is activated, either a point value MAX indicated by the maximum pointer 82 or a point value SEQ indicated by the sequential counter 83 is selected. One of the address registers 50 to 57 is selected by a selected point value SEL and the row address XADD stored in the selected one of the address registers 50 to 57 is output as the row address VADD. The value of one of the counter circuits 60 to 67 corresponding to the selected point value SEL is reset to an initial value.

As shown in FIG. 5, the control circuit 80 further includes a selection signal generator 84, a multiplexer 85, and a comparing circuit 86. The selection signal generator 84 generates a selection signal M/S having a value inverted each time the refresh selection signal RHR1 is activated twice. The selection signal M/S is supplied to the multiplexer 85. The multiplexer 85 selects the point value MAX output from the maximum pointer 82 when the selection signal M/S has one logical level (a low level, for example), and selects the point value SEQ output from the sequential counter 83 when the selection signal M/S has the other logical level (a high level, for example). The point value MAX or SEQ selected by the multiplexer 85 is output as the point value SEL. The point value SEL is used to select one of the address registers 50 to 57.

The point value MAX and the point value SEQ are compared with each other by the comparing circuit 86. When the point value MAX and the point value SEQ match with each other the comparing circuit 86 activates a skip signal SKIP The sequential counter 83 performs a count-up operation in response to the skip signal SKIP and the selection signal M/S.

FIG. 6 is a timing chart for explaining an operation of the control circuit 80.

In the example shown in FIG. 6, the refresh selection signal RHR1 is activated twice at each of times t1 to t17. When the refresh selection signal RHR1 is activated twice, the selection signal M/S is inverted. Therefore, the multiplexer 85 alternately selects the point value MAX and the point value SEQ. In the example shown in FIG. 6, the point value MAX is selected when the selection signal M/S is at a low level and the point value SEQ is selected when the selection signal M/S is at a high level. Accordingly, the point value SEQ is selected at the times t1, t3, t5, t7, t9, t11, t13, t15, and t17, and the point value MAX is selected at the times t2, t4, t6, t8, t10, t12, t14, and t16. The point value SEQ is incremented in response to a falling edge of the selection signal M/S. The point value SEQ is, for example, a 3-bit signal and is incremented from 0 to 7 and then returns to 0. The point value MAX is also, for example, a 3-bit signal and indicates the address of one of the counter circuits 60 to 67 currently having a greatest count value.

When the point value MAX and the point value SEQ match with each other, the comparing circuit 86 activates the skip signal SKIP. In the example shown in FIG. 6, the point value MAX and the point value SEQ both indicate a value “4” at the time t6. Because a count signal UP is activated in response thereto, the point value SEQ indicated by the sequential counter 83 is immediately incremented to “5”. As a result, the point value SEL generated when the refresh selection signal RHR1 is activated next time indicates a value “5” and “4”, which is the value of the point value SEL generated in response to the previous point value MAX, is not repeatedly output. Similarly, the point value MAX and the point value SEQ both indicate a value “0” at the time t12. The count signal UP is activated in response thereto and accordingly the point value SEQ indicated by the sequential counter 83 is immediately incremented to “1”. As a result, the point value SEL generated when the refresh selection signal RHR1 is activated next time indicates a value “1” and “0”, which is the value of the point value SEL generated in response to the previous point value MAX, is not repeatedly output. In this way, according to the present embodiment, the value of the point value SEL generated in response to the previous point value MAX and the value of the point value SEL generated in response to the current point value SEQ do not match with each other Therefore, any unnecessary row hammer refresh operation can be avoided.

Meanwhile, even when the point value MAX and the point value SEQ match with each other, the point value SEQ of the sequential counter 83 is not skipped if the point value MAX is selected next. For example, although the point value MAX and the point value SEQ both indicate a value “2” at the time t15, the count signal UP is not activated because the selection signal M/S is at a high level at this timing That is, the point value SEQ is kept at the value “2” and any unnecessary skip operation is not performed.

As explained above, in the present embodiment, the point value SEQ is skipped when the point value MAX and the point value SEQ match with each other. Therefore, the value of the point value SEL generated in response to the previous point value MAX and the value of the point value SEL generated in response to the current point value SEQ do not match with each other and any unnecessary row hammer refresh operation can be avoided.

In the present embodiment, the refresh operation may be performed plural times in the semiconductor device 10 in response to one refresh command issued from outside. In FIGS. 7A and 7B, examples where the refresh operation is performed five times each time the refresh signal AREF is activated once are shown. In the example shown in FIG. 7A, the refresh signal AREF is activated at each of times t20 to t24 and the internal refresh signal IREF is activated five times in a row each time the refresh signal AREF is activated. Because the refresh state signal RHR State is in an inactive state at the time t20, the refresh selection signal NR is activated synchronously with the internal refresh signal IREF. In this case, the refresh operation is performed sequentially on five normal refresh addresses NRADD in response to the refresh signal AREF. At the time t21, the refresh state signal RHR State is an active state. In the example shown in FIG. 7A, the refresh selection signal RHR1 is also activated synchronously with the internal refresh signal IREF during a period when the refresh state signal RHR State is activated. In the example shown in FIG. 7A, the refresh operation is performed on 12 row hammer refresh addresses +1 ADD, −1ADD in one row hammer refresh operation. In this case, the refresh operation cannot be performed on 12 addresses in response to one refresh signal AREF. Therefore, the refresh state signal RHR State is kept in the active state until the row hammer refresh operation to the 12 addresses is completed. That is, five row hammer refresh operations are performed in response to each of the refresh signals AREF activated at the time 121 and the time t22, and two row hammer refresh operations are performed in response to the refresh signal AREF activated at the time t23.

In the example shown in FIG. 7A, first and second row hammer refresh operations are respectively performed on +1ADD and −1ADD of the row address VADD indicated by the point value MAX. Third and fourth row hammer refresh operations are respectively performed on +1ADD and −1ADD of the row address VADD indicated by the point value SEQ. Fifth and sixth row hammer refresh operations are respectively performed on +1ADD and −1ADD of the row address VADD indicated by the point value MAX. Seventh and eighth row hammer refresh operations are respectively performed on +1ADD and −1ADD of the row address VADD indicated by the point value SEQ. Ninth and tenth row hammer refresh operations are respectively performed on +1ADD and −1ADD of the row address VADD indicated by the point value MAX. Eleventh and twelfth row hammer refresh operations are respectively performed on +1ADD and −1ADD of the row address VADD indicated by the point value SEQ. In this way, each time the refresh selection signal RHR1 is activated twice, the row hammer refresh operation corresponding to the point value MAX and the row hammer refresh operation corresponding to the point value SEQ are alternately performed.

In the example shown in FIG. 7B, the refresh signal AREF is activated at each of times t30 to t34. At the time t30, because the refresh state signal RHR State signal is in an inactive state, the refresh selection signal NR is activated. In this case, the refresh operation is performed sequentially on five normal refresh addresses NRADD synchronously with the internal refresh signal IREF. At the time t31, the refresh state signal RHR State switches to an active state. In the example shown in FIG. 7B, during a period when the refresh state signal RHR State is activated, the refresh selection signal RHR1 or RHR2 is activated. In the example shown in FIG. 7B, the refresh operation is performed on 12 row hammer refresh addresses +1ADD, −1ADD and the refresh operation is performed to four row hammer refresh addresses +2ADD, −2ADD in one row hammer refresh operation. In this case, because the refresh operation cannot be performed on 16 addresses in response to one refresh signal AREF, the refresh state signal RHR State is kept in the active state until the row hammer refresh operation to the 16 addresses is completed. That is, five row hammer refresh operations are performed in response to each of the refresh signals AREF activated at the time t31, the time 132, and the time 133 and one row hammer refresh operation is performed in response to the refresh signal AREF activated at the time t34. In response to the refresh signal AREF activated at the time t31, the refresh operation is performed on two row hammer refresh addresses +1ADD, −1ADD and the refresh operation is performed on three row hammer refresh addresses +2ADD, −2ADD. In response to the refresh signal AREF activated at the time t32, the refresh operation is performed on four row hammer refresh addresses +1ADD, −1ADD and the refresh operation is performed on one row hammer refresh address +2ADD, −2ADD.

In the example shown in FIG. 7B, first to fourth row hammer refresh operations are respectively performed on +2ADD, −2ADD, +1ADD, and −1ADD of the row address VADD indicated by the point value MAX. Fifth to eighth row hammer refresh operations are respectively performed on +2ADD, −2ADD, +1ADD, and −1ADD of the row address VADD indicated by the point value SEQ. Ninth and tenth row hammer refresh operations are respectively performed on +1 ADD and −1ADD of the row address VADD indicated by the point value MAX. Eleventh and twelfth row hammer refresh operations are respectively performed on +1ADD and −1ADD of the row address VADD indicated by the point value SEQ. Thirteenth and fourteenth row hammer refresh operations are respectively performed on +1ADD and −1ADD of the row address VADD indicated by the point value MAX. Fifteenth and sixteenth row hammer refresh operations are respectively performed on +1ADD and −1ADD of the row address VADD indicated by the point value SEQ.

The sequential counter 83 does not need to be a counter circuit that simply increments the point value SEQ, and can be a linear feedback shift register (LFSR) circuit that generates a pseudorandom number.

FIG. 8 is an example of a circuit diagram of an LFSR circuit 90. In the example shown in FIG. 8, the LFSR circuit 90 includes a shift register constituted by three flip-flop circuits 91 to 93, and an EXOR circuit 94. The flip-flop circuits 91 to 93 are cascade-connected and respective output bits B1 to B3 constitute the point value SEQ being a pseudorandom number. The bit B2 and the bit B3 are input to the EXOR circuit 94 and an output thereof is fed back to the flip-flop circuit 91 at the first stage. The count signal UP is supplied in common to clock nodes of the flip-flop circuits 91 to 93. A reset signal RESET is supplied in common to reset or set nodes of the flip-flop circuits 91 to 93. Accordingly, when the reset signal RESET is activated, the value of the point value SEQ is initialized to 4. When the count value UP is activated, the value of the point value SEQ is sequentially updated.

Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the an that the inventions extend beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the inventions and obvious modifications and equivalents thereof. In addition, other modifications which are within the scope of this invention will be readily app rent to those of skill in the an based on this disclosure. It is also contemplated that various combination or sub-combination of the specific features and aspects of the embodiments may be made and still fall within the scope of the inventions. It should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the disclosed invention. Thus, it is intended that the scope of at least some of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above. 

1. An apparatus comprising: a plurality of address registers each configured to store an address signal; a plurality of counter circuits each configured to store a count value corresponding to an associated one of the address registers; a first circuit configured to cyclically select one of the address registers in response to a first signal; a second circuit configured to select one of the address registers based on a count value of each of the counter circuits; and a third circuit configured to activate a second signal when the first and second circuits select a same one of the address registers.
 2. The apparatus of claim 1, wherein the first circuit updates a selection of the address registers in response to the first and second signals.
 3. The apparatus of claim 1, wherein the second circuit selecting one of the address registers that the count value corresponding thereto indicates a greatest value among the counter circuits.
 4. The apparatus of claim 1, further comprising: a fourth circuit comparing an input address with the address signal stored in each of the address registers; and a fifth circuit updating the count value of a corresponding one of the address registers when the fourth circuit detects that the input address matches with one of the address signals.
 5. The apparatus of claim 4, wherein the input address is overwritten in one of the address registers when the fourth circuit detects that the input address does not match with any of the address signals.
 6. The apparatus of claim 5, wherein the input address is overwritten in one of the address registers that the count value corresponding thereto indicates a smallest value among the counter circuits.
 7. The apparatus of claim 6, wherein the count value of the counter circuit corresponding to the address register storing the input address is reset to an initial value.
 8. The apparatus of claim 1, wherein the first and second circuits alternately select one of the address registers.
 9. The apparatus of claim 1, further comprising an address convertor that generates a refresh address based on the address signal stored in a selected one of the address registers by the first or second circuit.
 10. The apparatus of claim 1, wherein the count value corresponding to the selected one of the address registers is reset to an initial value.
 11. The apparatus of claim 1, wherein the first circuit includes another counter circuit that selects one of the address registers, and wherein a count value of the another counter circuit is updated in response to the first signal.
 12. The apparatus of claim 1, wherein the first circuit includes a linear feedback shift register circuit that selects one of the address registers, and wherein a count value of the linear feedback shift register circuit is updated in response to the first signal.
 13. An apparatus comprising: a plurality of address registers including at least first, second, and third address registers each storing an address signal; a first circuit configured to select one of the address registers in response to a first signal; and an address convertor configured to generate a refresh address based on the address signal stored in a selected one of the address registers by the first circuit, wherein the first circuit is configured to select the first, second, and third address registers in this order when a second signal is inactivated, and wherein the first circuit is configured to select the first and third address registers in this order without selecting the second address register when the second signal is activated.
 14. The apparatus of claim 13, further comprising a second circuit selecting one of the address registers based on count values assigned to the address registers, wherein the address convertor generates the refresh address based on the address signal stored in the selected one of the address registers by the first or second circuit.
 15. The apparatus of claim 14, wherein the first and second circuits alternately select one of the address registers.
 16. An apparatus comprising: a plurality of address registers each configured to store an address signal; first and second circuits configured to alternately select one of the address registers; and a third circuit configured to prevent the first circuit from selecting a first address register included in the address registers when the second circuit selects the first address register immediately before.
 17. The apparatus of claim 16, wherein the third circuit does not prevent the second circuit from selecting the first address register when the first circuit selects the first address register immediately before.
 18. The apparatus of claim 17, wherein the first circuit cyclically selects one of the address registers.
 19. The apparatus of claim 18, wherein the second circuit selects one of the address registers based on count values assigned to the address registers.
 20. The apparatus of claim 19, further comprising an address convertor that generates a refresh address based on the address signal stored in a selected one of the address registers by the first or second circuit.
 21. An apparatus comprising: a plurality of address registers; a plurality of counter circuits provided correspondingly to the plurality of address registers, each of the plurality of counter circuits being configured to update a count value in response to detect an access to an address signal stored in an associated one of the plurality of address registers; a first circuit configured to select one of the plurality of address registers based on the count values of the plurality of counter circuits; a second circuit configured to select one of the plurality of address registers irrespective of the count values of the plurality of counter circuits; and a third circuit configured to activate a skip signal when the same one of the plurality of address registers is consecutively selected by the first circuit and the second circuit.
 22. The apparatus of claim 21, wherein the second circuit is configured to cyclically select one of the plurality of address registers.
 23. The apparatus of claim 21, wherein the first circuit is configured to select one of the address registers that the count value corresponding thereto indicates a greatest value among the counter circuits.
 24. The apparatus of claim 21, wherein the first and second circuits are configured to alternately select one of the address registers.
 25. The apparatus of claim 21, wherein the count value corresponding to the selected one of the address registers is reset to an initial value. 